Microwave circuits, often also referred to as millimeter wave circuits, require passive components in addition to active microwave components. Capacitors or coils, which are used to construct filters, are referred to, for example, as passive components.
Furthermore, resonators are required to select a narrow high-frequency band from the frequency spectrum emitted by the active microwave component.
IMPATT diodes, Gunn diodes or fast transistors, in particular heterojunction bipolar transistors and high-frequency MOS transistors, are usually used as active microwave components.
During the operation of the microwave circuit at high frequencies, in particular above 50 GHz, parasitic effects are produced as a result of the geometry of the passive components. The result of these parasitic effects is that the coils and capacitors no longer behave like coils and capacitors.
For this frequency range, filters are mainly constructed as line length component. In this case, the line is interrupted to produce a gap. If the length of the gap in the propagation direction of a wave is .lambda./2, then a wave having the wavelength .lambda. is transmitted, whereas other wavelengths are reflected at the gap. The space requirement of line length components of this type is linked to the wavelength and hence to the frequency.
Slot-type resonators or planar disk-type resonators are used as resonators in this frequency range. Slot-type resonators again designate gaps in lines, which have an extent corresponding to an integer multiple of .lambda./2. For impedance reasons, gaps having an extent of integer multiples of A are preferable here. The dimensions of a slot-type resonator are therefore again coupled to the wavelength.
Planar disk-type resonators designate enlargements of the line cross-section which are in disk form. Resonance occurs for specific cross-sections of such disks.
Microwave circuits are mainly constructed using conventional stripline technology (see, for example, H. H. Meinel, Millimeter-Wave Technology advances since 1985 and Future trends, IEEE Transaction on microwave theory and techniques, Vol. 39, No 5, 1991, pages 759 to 767). In stripline technology, the electromagnetic field propagates between a stripline and a grounded metal plate. The grounded metal plate and the stripline are in this case arranged on opposite surfaces of an insulating substrate, for example of a printed circuit board. The packing density of the striplines is in this case limited by lateral field components. Furthermore, the impedance of the stripline is greatly dependent on the substrate thickness, with the result that fluctuations in the substrate thickness have a noticeably disturbing effect.
The problem of fluctuations in the substrate thickness is avoided by the use of coplanar lines, as is disclosed, for example, in D. F. Williams et al., Design and performance of coplanar wave guide bandpass filters, IEEE Transaction on microwave theory and techniques, Vol. MTT-31, No 7, 1983, pages 558 to 565. Lines which run between grounded lines are arranged on an insulating substrate, generally composed of high-resistance GaAs. The electromagnetic field in this case propagates between the line and the grounded lines. The substrate is essentially used for mechanical stabilization.
The lines are produced on GaAs substrates using electroplating technologies, which can be structured relatively coarsely. In this way, it is possible to produce structures having a structure size of greater than 5 .mu.m.
Hybrid superstructures are used for many high-frequency applications, for example Doppler radar.